- Disable Enhanced Tracking Protection in the little padlock (on the Firefox URL bar, not somewhere in uBlock Origin)
- If you've got a VPN, try disabling it. YouTube likes to be very heavy-handed with their blocking if I have my VPN on vs. not
- Clear your cache. For some reason this can help sometimes, idk why. Can be beneficial to sign out and in again, too. (try Firefox settings > Privacy & Security > Manage browsing data > search "youtube" and delete too.)
- Reload your uBlock filter lists in settings (un-select, then re-select)
- If you're not signed in, this'll make things a million times worse. YouTube, especially nowadays, loves to block clients without a login because they're often used by video downloaders, AI scrapers, etc.
Other than that, there's not much else. Sometimes switching to a chromium-based browser helps too, but I doubt you want to have to entirely switch browsers any time you want to watch a YouTube video.